Diode pumped solid state lasers are typically cooled by a flow of water or ethylene glycol water (EGW) mixture. This coolant must be delivered at a precise temperature so that the diode light will remain within a particular, narrow wavelength range. In many cases, the operation of the laser consists of short lasing durations with an order of magnitude of longer off times (e.g., on for 1 minute and off for 9 minutes). Therefore, a thermal energy storage (TES) device may be desirable to enable the use of smaller refrigeration systems or other heat rejection systems. The TES device could either employ a phase change material (PCM) or it could be just a tank of coolant, as a thermal mass. If the initial chill temperature of the coolant is as much as 15° C. to 20° C. colder than the laser inlet or exit temperature, then a water tank as a TES device becomes a particular attractive option from a weight basis. Other high power applications require similar coolant systems.
A typical cooling system would include a coolant delivery loop for delivering coolant during lasing durations, and a rechill loop for rechilling the coolant heated by the laser during the lasing duration. The cooling system can include a TES device that is coupled to both the laser flow loop and the rechill loop. The TES device in the coolant delivery loop can be plumbed with a bypass to a thermal control valve (TCV) that mixes hot coolant from the laser with cold coolant from the TES device to achieve a controlled, set temperature desired for the laser inlet flow. A portion of the hot coolant is also delivered to the TES device to push out the cold coolant to mix with the hot coolant. In the usual thermal analysis of the tank, the hot and cold coolant in the tank is assumed to be well mixed. However, in most tank configurations the hot coolant is injected into the tank through an inlet flow jet that bypasses a large portion of the cold coolant resulting in poor mixing of hot and cold coolant and inefficient use of the cold coolant.